Methods of and means for vacuum deposition

ABSTRACT

Method of coating a surface of an article comprises steps of carbonizing organic material to provide a flexible elongated electrically conductive heater element, forming a solid coating of a vaporizable coating material on said element, positioning the said article and heater element in a vacuum chamber with said heater element stretched between two spaced electrodes and heating said element by passing electrical current through said electrodes and heating element to vaporize the vaporizable coating thereon.

United States Patent Baxter Dec. 23, 1975 [5 METHODS OF AND MEANS FORVACUUM 3,607,368 9/1971 Van Amsteln: 117/107 DEPOSITION FOREIGN PATENTSOR APPLICATIONS [76] Inventor: Alexander Samuel Baxter, Broomhill 1613,113 1/1961 Canada e, at Brickhill, Bletchley, 964,461 7/1964 UnitedKingdom Buckmghamshire, England 996,338 6/1965 United Kingdom [22] FiledJan 15 1973 1,086,793 l0/l967 United Kingdom [211 App] 323 OTHERPUBLICATIONS n Holland, Vacuum Deposition of Thin Films, John RelatedApplication Data Wiley & Sons, 1956, pp. 108-117. [63]Continuation-impart of Ser. No, ll3,3l3, Feb. 8, I

Primary ExaminerHarris A. Pitlick [30] F A r r Pr t D ta Attorney,Agent, or Firm-Brisebois & Kruger orelgn pp ma non ion y a Feb. 12, 1970United Kingdom 6789/70 57 ABSTRACT 152 US. Cl. 427/132; ll8/49;1l8/49.l; Method of WW 3 article 9 71; 219/275; 427/248; 427/250- 427steps of carbomzmg organic material to provlde a flex- [511 t 2 U C23C13/02. C23C [3/04 ible elongated electrically conductive heater element,[58] Field of Search...., 117/106 1% 107 107 1 forming a Solid CoatingOf a vaporizable coating mate 7/1072 118/49 219/271 rial on saidelement, positioning the said article and 250 heater element in a vacuumchamber with said heater element stretched between two spaced electrodesand [56] References Cited heating said element by passing electricalcurrent UNITED STATES PATENTS through said electrodes and heatingelement to vaporize the va orizable coatin thereon. 2,930,879 3/1960Scatchard 117/107 X p g 2,996,412 8/1961 Alexander 117/107.1 x 4 Claims,6 Drawmg Figures US. Patent Dec.23,1975 Sheet10f2 3,928,659

US. Patent Dec. 23, 1975 Sheet20f2 3,928,659

METHODS OF AND MEANS FOR VACUUM DEPOSITION This basically simple processis one which has been known and practiced for many years; with somecoating materials, which have a low melting point or a high vaporpressure below the melting point and are not strongly reactive with thesurface of heating means or other materials in the vacuum, coatings canbe satisfactorily obtained. This is particularly the case where smallsingle articles or small areas of surfaces are to be coated, and wherethe circumstances of the coating operation, for example in laboratory orsmall scale conditions, permit a relatively inefficient or expensiveshort run process to be tolerated.

The problems of producing coating process which is efficient as tomaterial and, to a lesser extent, power consumption for the heatingmeans are substantial, and are greater with certain coating materials,and certain surface to be coated. For example there is a wide need forcoating aluminum on thin sheet thermoplastic material, such aspolyethylene terephthalate and polyvinyl sheet. Aluminum requires muchheat for vaporisation so that in these conditions there is danger ofsoftening or damaging the plastic sheet while it is adjacent the heater.

It is desirable to be able to run a vacuum coating apparatus for as longas possible on each load, and in this respect the nature and operationof the heating elements are important or dominating. The elementsnormally consist of a number of rods or bars of suitably refractorymaterial, through which the electric heating current has to be passed,disposed across the width of the web to be coated. The material to bedeposited is brought into contact with the surface of the heater. Thelife of such a heater is relatively short, often only a few hours, sincethe elements are consumed or eroded by the action of the coatingmaterial. The elements must be individually fed with coating material;the life of a set of elements is no longer than the life of theshortest, and failure of one element leads to non-uniformity of depositacross the width of the web. In consequence, it is necessary to removeand replace the elements at frequent intervals, re-making the necessaryelectrical connections to the replaced elements, and this is costly andtime consuming.

For use as heating elements carbon rods are attractive because of therefractory nature of the material and comparative cheapness, andelements consisting basically of carbon have been much used in practice.Tungsten as wire or rods can be used but also has short working life andis expensive.

In the past it has been suggested that carbon heaters in the form ofcarbon fibre woven material could be used, on which material to beevaporated, such as copper, is locally placed. Further reference will bemade'to this prior proposal.

The present invention has for its object to provide a vacuum depositionprocess which uses elements which are relatively cheap to manufactureand efficient in use both as to power consumption and efficiency ofdeposition.

The invention also has for its object to provide a vacuum depositionprocess which uses elements which permit extended runs, limited only bythe material to be coated.

THE INVENTION The invention is based upon the use of a heater elementcomposed of carbon fashioned into an elongated, flexible form, as athread or tape or the like in which the carbon thread or filaments orthe like extend along the length of the heater without any substantialdegree of transverse displacement. The heater carries upon it a quantityof the coating material the material is distributed over at least theheated length of the heater and conveniently over the continuous lengthof the heater and wholly covers the surface of the heater. By this meansa uniform coating material on the surface of the article is secured.

Further objects and advantages of the invention will appear from thefollowing description of embodiments thereof, given by way of example,in conjunction with the accompanying drawings.

THE DRAWINGS FIG. 1 is a diagrammatic side elevational view of a vacuumdeposition apparatus;

FIG. 2 is a diagrammatic sectional view of the same apparatus taken onthe line 22 of FIG. 1;

FIG. 3 is a sectional view of one form of heater element.

FIG. 4 is an enlarged detail of the heater element of FIG. 3.

FIG. 5 is a view of another form of heater element, and

FIG. 6 is a diagram showing an arrangement for coating both surfaces ofa web.

FIG. 1 shows, partly diagrammatically, an apparatus suitable for coatingwith metal one surface of a thin flexible plastics film 10, of, forexample, polyethylene terephthalate, The film passes froma supply reel 1l, on spindel 12, over rollers 13, 14 and 15 to take-up reel 16 onspindle 17. As shown, the film follows a U-shaped path between rollers13 and 15, round roller 14. Motors (not shown) are provided for rotatingspindles 12 and 17. The motor drives are reversible so that the film canbe wound and rewound from one reel to the other as may be desired.

The apparatus is mounted in a vacuum chamber. This can take variousforms but is shown as comprising a cylindrical body 20, closed at oneend by end plate 21 and at its other end by an end plate 22. Forconvenience, the end plate 22 can be mounted on a permanent structure,the cylindrical body 20 and end plate 21 being mounted together on amovable carriage 23. The body 20 and end plate 22 are held by detachableclamps 24. Carried on the fixed end plate 22 are the connections, suchas 24 to the vacuum pumping equipment, and connections 25 to theelectrical supply and control gear for controlling the various drivingmotors in the apparatus.

Heating element assemblies are mounted in the chamber. Normally, two ormore of such elements may be used, but as these are similar inconstruction, one

only will be described. One such assembly is shown in FIGS. 1 and 2.This comprises a main support plate 26 which has two spindles 27 and 28to receive respectively reels 29 and 30. A filamentary heater 31,initially wound on supply reel 29, passes between a pair of conductiverollers 32 and 33 and then extendsin' a substantially straight, free andunobstructed path across a cut out portion 26a of the plate 26, to passbetween the nip of a further pair of conductive rollers 34, 35 andthence to the take-up reel 30. The filament 31 can be advanced from reel29 to reel electrically under remote control, the rollers 34 and 35being driven by an electric motor 36 and reduction gear 37. The reel 30is driven by a takeup motor 38 which is lightly energised, or drivesthrough a friction means such as pad 39, to drive the take-up reel. Reel29 is under light frictional restraint, by resting on a suitablefriction pad 40.

Means are provided for passing an electric current through that part ofthe filament 31 which spans the rollers, across gap 26a. Connection ismade from the rollers 32, 33 and 34, 35 to electrical contacts on theplate 26, as at 41 and 42. Rollers 36 and 37 can be water cooled ifexperience shows this to be desirable. Connections to motors 36 and 38,are made by means of a plug and socket member 43.

The unit shown in FIG. 2 is capable of being readily inserted into andwithdrawn from the chamber. The edges of the board are engaged in guides44 and 45 carried on a support 46 from end plate 22; a suitable latch,can be provided for retaining the plate securely in position in theguides.

When the plate is thus positioned, the filament 31 will be positionedsubstantially mid-way between the two straight spans of the U-shapedportion of the film web 10. The cut-out 26a is of a depth such as topermit ready threading of the web when the apparatus is being preparedfor coating of a web of material. When the heater assembly is inserted,connections are made automatically to the heater terminal contacts 41and 42 for supplying a suitable current to heat the portion of thefilament 31 which extends across the cut-out 26a. If further heaterelement assemblies are used, they can be positioned on the same oropposite sides of the chamber, with the filaments in a central positionbetween the spans of the web.

The heater filament 31 is of a nature such that when it is raised tooperating temperature, the desired material for deposition is vaporisedfrom the surface of the filament.

In general, this requirement can be met by a wide variety of elements ofdifferent shapes, constructions and materials, the most suitable elementdepending upon circumstances, but the element must be conductive, eithersuperficially or throughout, and must be capable of withstanding theconditions, including temperature, existing in the vacuum chamber duringvaporisation. For the purpose of the present invention it is necessarythat the element should be sufficiently flexible, to allow the elementto be coiled.

The nature of the material of the heater is an important feature of thepresent invention and before proceeding to describe the construction ofthe heater it is appropriate to consider the operation of a heater whenbeing used to vaporise a metal in the conditions of vacuum deposition.It is manifest that the heater will be raised to a temperature at whichvaporisation takes place, but the conditions that obtain when thistemperature is reached vary considerably with different depositionmaterials and different heater surface composition.

An important consideration is whether the molten material wets thesurface of the heater. If the material does not wet the heater surfacethen when the material has become molten it will run into globules. Whenthis happens it is detrimental to successful commercial coating forseveral reasons.

An important reason is that when globules are formed, vaporisation takesplace from a series of localised areas, that is, the globules. There islittle control of the location of such spots and they may changelocation rapidly and unpredictably; the vaporisation is violent oreruptive. This localised evaporisation is conducive to lack ofuniformity of the vaporisation and lack of control of the thickness ofthe deposited layer of material.

Further, because of the localisation of 'the globules, the rate of heatloss from the surface of the heater is irregular. In vacuum, the energydissipated from a unit surface areas of the heater is dissipatedsubstantially entirely by two modes only: by radiation as heat, and bytransfer to the molten material to supply the latent heat ofvaporisation of the material; there may be a degree of terminalconduction. With a substantially uniform heater heated by current passedthrough it there will be a substantially uniform dissipation of energyper unit length of the heater, so that more heat will have to bedissipated as radiation from the heater surface between the globulesthan from the globule-covered surface areas. Not only does this radiatedenergy present a direct wastage of the power fed to the heater, but theradiated energy will uselessly and sometimes detrimentally heat thenearby surfaces, including the deposition surface. With plastic films,such heating is very undesirable.

The eruptive vaporisation of the material also conduces to inefficientdeposition, and less of the vapor is deposited on the surface of thearticle. This leads to wastage of the coating material, which may or maynot be economically tolerable, but it also results in deposits on partsof the apparatus, calling for more frequent cleaning, and resultant downtime.

With the invention the heater element is composed of carbon fashionedinto a filament, which may be a thread or tape, which is flexible. Thematerial is applied to the element before the element is heated tovaporising temperature. By making the element of fine carbon fibres itwill be given an adequate flexibility.

The degree of flexibility required is that the heater must be capable ofbeing coiled and handled within the vacuum chamber without adverselyaffecting its mechanical and physical properties. As will appear, theuse of a multifilament carbon fibre heater is advantageous in this aswell as other respects. Suitable carbon fibres can be obtained by thecarbonising appropriate organic compounds, in known manner; a largenumber of the fine diameter fibres are combined mechanically as bytwisting, before carbonising the organic material of the fibres. Goodresults have been obtained by carbonising a material sold, for domesticknitting, as double ply material, under the trade mark Courtelle. Thisconsists of artificial fibres, about 6 microns in diameter; aftercarbonising in known manner, the carbonised thread is aboutone-sixteenth inch in diameter. The fibres are staple fibres, with onlya small degree of twist, so that the fibres all lie substantially along,or

only at a small angle to the length of the thread. A flat tape materialmade up of threads of this kind side" by side, is particularly suitable.It is important that the length-wise fibres of the filament should bedisposed substantially along the length of the filament withouttransverse displacement such as occurs, for example, with a conventionaltextile weave, where the warp threads are bent by the weft threads. Thepurpose of this is that the heater should be dimensionally stable, inthe direction along its length, when heated.

The heater carries on or near its surface region the material to bevaporised; the material can be embodied by a mechanical or chemicalmeans on the surface of the element by alloying or by forming acompound. which is pyrolysable, but in any case the material is appliedwholly over the surface of the heater. A heater element of this kind isshown in diagrammatic form, in FIGS. 3 and 4. In FIG. 3 the fourfilaments 50 are comprised of individual fibres 51; the adjacent edgesof the four filaments can be made to adhere sufficiently to form thetape by light combing, whilst preserving substantially longitudinaldisposition of the fibres.

The coating on the individual fibres is shown at 52 in FIG. 4. It willbe appreciated that the fibre and coating dimensions shown in FIGS. 3and 4 are not to scale.

In some cases, material to be vaporised can be applied to the heaterfilament by a conventional coating method, including plating byelectrolytic, electroless or mechanical plating. The material can alsobe applied to the filament as a powder coating, the filament being firsttreated with a suitable adhesive, or as a paint in suitable solvent orvehicle. The process used for applying the coating must preserve theflexibility of the material not only in the cold state, but also whenheated, and the heater must remain dimensionally stable in the heatedcondition. It is for this reason that the lengthwise fibres should beaxially disposed as described, for then the heater will retain itslengthwise dimension and will not sag or contract. This also precludesthe use of a conventionally woven material where the warp threads yieldto the weft threads. Such a material is not sufficiently stable whenheated. Any deformation may cause the coating to become detached fromthe heater.

The coating material must not wet the surface of the heated element.However, by restricting the quantity of material on the heater thepossiblity of the molten material running into droplets or blobs asdescribed is obviated. By using a large number of fibres to form thefilament heater, the quantity of coating material, within this limit,that can be used is increased. As the material on the heater reaches itsmelting point it will tend to form globules, but the globules are formedin initially a very small size, and there is insufficient material topermit aggregation of the small globules to form the large and localiseddetrimental globules as described above. The heat radiation ismaintained substantially uniform and highly uniform vaporisation overthe heated length of the heater takes place.

The very large numbers of very small diameter fibres make it extremelydifficult to estimate the coating thickness, and the high temperatureconditions in which the coating forms the globules and the fact thatglobulation is followed rapidly by evaporation, makes equally difficultan estimation of globule size. It is clear however by empirical methodsto determine if the coating is of an excessive thickness then theglobules, of excessive size, give rise to visible nonuniformity ofheating or deposition or both.

In general, however, it is estimated that the coating will be only a fewmicrons thick, and the globules only a few microns in diameter.

In a practical case, using a tape made of six threads as described, asuitable weight of coating has been estimated to be to mg per square cm.of heater surface.

Materials which are not electrically conductive can be used by applyingthe coating to the heater so as to leave the conductive heater exposedat intervals. Thus, in FIG. 5 the heater 31 is shown diagrammatically ascoated with material 53, leaving gaps at 31a at which contact can bemade to the conductive heater surface by the contact rollers. In thiscase, the element is advanced step by step.

The process and apparatus described make possible improvements in otherrespects. For example by using a plurality of heaters with differentevaporation materials composite deposits can be produced; such depositsmay be homogeneous or may be in the nature of successive layers ofdifferent materials.

The coating material should not wet the heater, for if this occurs thereis brought about a mechanical deformation which may be severe, or verysevere. If the heater is wetted, the surface tension effect is greatenough to distort the heater and affect vaporising operation. Uniformityof vaporisation or control, may be lost.

The invention can be used to produce magnetic coatings on base material,suitable for use in magnetic recording, by deposition of magneticmaterial including iron, cobalt, nickel and chromium, deposited asmetals, or alloys, or solid solutions of such metals. The alloys orsolid solutions can best be deposited directly from a plurality ofheaters. When such materials are deposited, the deposit is initially ofa very fine structure, such that the degree of resolution, as'a magneticrecording medium, is very high, but the film is extremely thin. If thefilm thickness is increased, the magnetic particles agglomerate and thefineness of structure is diminished.

However, if a layer of non-magnetic material is deposited over the firstmagnetic layer, before the second deposition is made, the resolution isretained and the volume of magnetic material is increased. A verysuitable non-magnetic material is nylon, having good antifrictionproperties which are advantageous when contact-type magnetic heads areused. There may be as many repetitions of deposition steps as may bedesired, the film being wound and rewound between reels 11 and 16 asnecessary, by means of the reversible drive large supply and take upreels for the heater element wire, with sufficient wire to last for manyoperations of the apparatus. In this case, the removable elementassembly will not offer any further substantial economic advantage.

Both sides of a web can be coated, with an arrangement as shown in FIG.5; filament heaters 31, 31a disposed between parallel spans of the web10. Two or more webs can be coated simultaneously for example to avoidcontact of the coating surface of the web with roller 14, by usingsuitable guide roller arrangements.

The invention has been found to make a major improvement in vacuumdeposition giving a much higher output from a given equipment and aproduct of improved uniformity. A wider range of materials, under closercontrol, can be deposited.

I claim:

1. A method of coating a surface of an article by the evaporation of avaporizable coating material in a vac- 'uum chamber, which methodcomprises the steps of carbonizing or graphitizing organic material toprovide a flexible elongated electrically conductive heater element inthe form of a tape which is inextensible under the conditions to whichit is subjected in carrying out said method, forming a solid coating onthe surface of said element with a quantity of said vaporizable materialsufficient to substantially wholly cover the surface of said heater butinsufficient to permit the formation of globules of said coatingmaterial more than a few microns in size, said vaporizable materialhaving the property of not wetting the heater when at the vaporizingtemperature of said material, said coated heater 8 element retaining itsflexibility when coated, disposing said article and said element withsaid material thereon in said chamber, providing electrical connectionsto regions of said element spaced along its length, heating that part ofthe element between said connections by passing electrical currentthrough said connections and said part to bring said part of the elementto a temperature which causes uniform vaporization of the material fromsubstantially the full length of said part and deposition of saidvaporized material on said surface of the article and, while maintaininga vacuum in said chamber, moving said element lengthwise to bring adifferent part of the element between said connections, and heating saiddifferent part.

2. A method as claimed in claim 1 in which said tape is fibrous.

3. A method as claimed in claim 1 in which said vaporizable material isa metal.

4. A method as claimed in claim 3 in which said metal is a magneticmetal.

1. A METHOD OF COATING A SURFACE OF AN ARTICLE BY THE EVAPORATION OF AVAPORIZABLE COATING MATERIAL IN A VACUUM CHAMBER, WHICH METHOD COMPRISESTHE STEPS OF CARBONIZING OF GRAPHITIZING ORGANIC MATERIAL TO PROVIDE AFLEXIBLE ELONGATED ELECTRICALLY CONDUCTIVE HEATER ELEMENT IN THE FORM OFA TAPE WHICH IS INEXTENSIBLE UNDER THE CONDITIONS TO WHICH IT ISSUBJECTED IN CARRYING OUT SAID METHOD, FORMING A SLID COATING ON THESURFACE OF SAID ELEMENT WHICH IS QUANTITY OF SAID VAPORIZABLE MATERIALSUFFICIENT TO SUBSTANTIALLY WHOLLY COVER THE SURFACE OF SAID HEATER BUTINSUFFICIENT TO PERMIT THE FORMATION OF GLOBULES OF SAID COATINGMATERIAL MORE THAN A FEW MICRONS IN SIZE, SAID VAPORIZABLE MATERIALHAVING THE PROPERTY OF NOT WETTING THE HEATER WHEN AT THE VAPORIZINGTEMPERATURE OF SAID MATERIAL, SAID COATED HEATER ELEMENT RETAINING ITSFLEXIBILITY WHEN COATED, DISPOSING SAID ARTICLE AND SAID ELEMENT WHICHSAID MATERIAL THEREON IN SAID CHAMBER, PROVIDING ELLECTRICAL CONNECTIONSTO REGIONS OF SAID ELEMENT SPACED ALONG ITS LENGTH, HEATING THAT PART OFTHE ELEMENT BETWEEN SAID CONNECTIONS BY PASSING ELECTRICAL CURRENTTHROUGH SAID CONNECTIONS AND SAID PART OF BRING SAID PART OF THE ELEMENTTO A TEMPERATURE WHICH CAUSES UNIFORM VAPORIZATION OF THE MATERIAL FROMSUBSTANTIALLY THE FULL LENGTH OF SAID PART AND DEPOSITION OF SAIDVAPORIZED MATERIAL ON SAID SURFACE OF THE ARTICLE AND, WHILE MAINTAININGA VACUUM IN SAID CHAMBER, MOVING SAID ELEMENT LENGTHWISE TO BRING ADIFFERENT PART OF THE ELEMENT BETWEEN SAID CONNECTIONS, AND HEATING SAIDDIFFERENT PART.
 2. A method as claimed in claim 1 in which said tape isfibrous.
 3. A method as claimed in claim 1 in which said vaporizablematerial is a metal.
 4. A method as claimed in claim 3 in which saidmetal is a magnetic metal.